Antigen Availability and DOCK2-Driven Motility Govern CD4⁺ T Cell Interactions with Dendritic Cells In Vivo

Parenchymal migration of naive CD4⁺ T cells in lymph nodes (LNs) is mediated by the Rac activator DOCK2 and PI3Kγ and is widely assumed to facilitate efficient screening of dendritic cells (DCs) presenting peptide-MHCs (pMHCs). Yet how CD4⁺ T cell motility, DC density, and pMHC levels interdependently regulate such interactions has not been comprehensively examined. Using intravital imaging of reactive LNs in DC-immunized mice, we show that pMHC levels determined the occurrence and timing of stable CD4⁺ T cell-DC interactions. Despite the variability in interaction parameters, ensuing CD4⁺ T cell proliferation was comparable over a wide range of pMHC levels. Unexpectedly, decreased intrinsic motility of DOCK2^-/- CD4⁺ T cells did not impair encounters with DCs in dense paracortical networks and, instead, increased interaction stability, whereas PI3Kγ deficiency had no effect on interaction parameters. In contrast, intravital and whole-organ imaging showed that DOCK2-driven T cell motility was required to detach from pMHC^low DCs and to find rare pMHC^high DCs. In sum, our data uncover flexible signal integration by scanning CD4⁺ T cells, suggesting a search strategy evolved to detect low-frequency DCs presenting high cognate pMHC levels.